Femtosecond laser source for real-time atmospheric gas sensing in the UV–visible

We propose an experimental method for detecting molecules in the UV–visible range using ultrashort laser pulses. Two types of sources are used: a continuum generated by 200 kHz Ti:sapphire regenerative amplifier system extending from 320 to 1100 nm, and a near-gaussian femtosecond pulse (100 fs) generated by an optical parametric amplifier. Both broadband sources allow the real-time detection of the oxygen, the nitrogen dioxide NO₂ and the water vapor bands. Moreover, the concentration of NO₂ can be determined within the 10 ppb sensitivity range by using a specific nonlinear fit technique.     

Narrow-band seeding of an optical parametric amplifier in the femtosecond regime

Nearly transform limited femtosecond pulses tunable between 2.56 and 3.16 m have been generated by optical parametric amplification. The single stage parametric amplifier is pumped by a tunable high power femtosecond Ti:sapphire laser system at 1 kHz repetition rate and seeded by quasi-continuous wave (cw) radiation from the Q-switched Nd:YLF laser used to pump the regenerative amplifier. The 100 fs idler pulses are shorter than the pump pulses. The mechanism of the achieved pulse compression is discussed and experimental results are compared with numerical simulations.     

Modelling of 980 nm pumped EDWAs: Spectroscopic variations associated to fabrication process

In this work the optical amplification, in the small signal regime, at 1.5 m in Zn-diffused, proton exchange and reverse proton exchange Er³⁺-doped waveguide amplifiers (EDWAs) based on LiNbO₃ under 980 nm excitation has been modelled, including the dominant up-conversion channel activated by this pumping wavelength. The overlapping integrals method has been used to investigate how the spectroscopic changes, induced by the waveguide fabrication technique, affect the optical amplifier performance. In particular, it has been found that either the amplifier threshold and the maximum gain achievable are spectroscopy-dependent and therefore fabrication-dependent.     

Compact positron beam for measurement of transmission moderator efficiencies and positron yields of encapsulated sources

We have constructed a simple positron beam for measuring both the efficiencies of positron moderator foils and the positron yields of encapsulated positron beam sources. The slow-positron emission rate from the moderator is determined from the positron annihilation radiation. This technique is also applied to measure source yields. In addition to the coincidence technique, the source yield is determined by measuring electric currents generated by the annihilating positrons. The performance of the system is demonstrated with different tungsten moderator foils before and after heat treatments, and several sources with primary activities in the range 1-50 mCi.     

A Method for Separating the Point-Source Foreground from the Cosmic Microwave Background Maps

We propose a new technique for high-accuracy reconstruction of the cosmic microwave background (CMB) anisotropy from observations contaminated by unresolved extragalactic point sources and pixel noise. The fundamental difference of this technique from the methods suggested before consists in reconstructing the CMB temperature fluctuations in regions contaminated by point sources with allowance for a priori information on the boundedness and spherical symmetry of the spatial power spectrum of the CMB anisotropy, which obeys the Gaussian statistics. The problems of deconvolving the initial maps, localizing point sources, and interpolating the CMB values in the holes are solved by using effective modifications of the well-known maximum entropy method. The results of simulations demonstrate the possibility of achieving the highest accuracy.     

Analytical expression of the relations between the output characteristics of an AuSi-spectrometer and the detector and amplifier characteristics

Analytical relations between characteristics describing the behaviour of a peak generated by a beam of monoenergetic particles and parameters characterizing the working conditions in the spectrometer from the beginning of the collection of carriers generated by the detected particle in the detector up to the output from the shaping amplifier are derived for a spectrometer with a surface barrier silicon detector. These relations are then experimentally tested for alpha particles and conversion electrons.This study was performed as part of the IAEA Co-ordinated Research Programme (the IAEA Research Contract of the Institute of Organic Chemistry and Biochemistry, Czechoslovak Academy of Sciences, No. 3673/RB).The authors wish to thank Ing. Jií Fiala and Mr. Vladislav Pánek for their technical assistance in the measurements and in preparing the radiation sources.     

A 44 GHz HEMT amplifier

A 44-GHz amplifier using 0.25-m gate length and double-heterojunction structure HEMT devices is described. Higher gain and power performance have been obtained from the amplifier using this device at millimeter-wave frequencies. A spot gain of 9.4 dB and a 1-dB gain compression point of +7.5 dBm has been achieved at 43.5 GHz.This work was supported by the Air Force Space Division under Contract No. F04701-84-C-0113.     

An alternative method for the determination of element concentrations in schizophrenic, lung cancer and leukaemia patient bloods

Concentration of the elements present in schizophrenic, lung cancer and leukaemia patients’ bloods were determined using energy-dispersive X-ray fluorescence (EDXRF) method. EDXRF spectrometer with an annular and sources was applied for the analysis of blood samples. A sample preparation procedure suitable for the EDXRF, the experimental approach, analytical method used in this study and the results were presented. EDXRF technique has been successfully used for the determination of elements present in blood.     

Real-time autocorrelator with enhanced temporal scan range for measurements of ultrashort laser pulses

A simple, low-cost light emitting diode-based quasi-real-time autocorrelator has been developed with two-fold enhancement in temporal scan range than conventional commercial autocorrelators. We use the technique of twin audio-speaker, but avoid the complex electronic circuitry to achieve two 180° out-of-phase ramp signals. Next, the use of pick-up coils, switching electronics and sum amplifier to generate a time marker signal for autocorrelation signal measurement is also dispensed with. The suitability of our setup is demonstrated by recording autocorrelation signals of 200 fs laser pulses from Nd:phosphate oscillator under different ramp signal voltages.     

Laser-diode-seeded single and double stage femtosecond optical parametric amplification in the mid-infrared

Tunable mid-infrared pulses are generated with a MgO:LiNbO₃-based traveling-wave optical parametric amplifier in single and double stage operation regime pumped by a 1-kHz Ti:sapphire regenerative amplifier system. For the first time to our knowledge a pulsed laser diode is successfully used in the femtosecond regime as a compact seed source at the signal wavelength where the tunability of the generated idler pulses can be realized by tuning the seed wavelength. Almost transform limited mid-infrared pulses as short as 130 and 160 fs with pulse energies as high as 5 and 15 J are produced near 3.5 m by the single and double stage scheme, respectively. The pulse to pulse fluctuations in both cases do not exceed 2% which corresponds to the stability of the pump source.     

A method to improve the quality of silica nanoparticles (SNPs) over increasing storage durations

The solvent varying technique (SVT) provides a simple method for the production of uniform batches of silica nanoparticles (SNPs) of a target average diameter. SNPs synthesized using the SVT have been observed to agglomerate over increasing storage times leading to an increase in average particle diameter. Since the particle diameters of the SNPs produced using the SVT may vary over increasing storage durations, the previous model, suggested by Gao et al., which is based on the diameter of the original SNPs, is unreliable when predicting a target particle diameter using the initial volume of ethanol. A centrifuge and replacement of solvent method has been applied in this investigation to the SNP solutions created using the SV technique. This reduces the amount of unused reactants in the centrifuged colloidal suspensions, which further improves the quality of the SNPs and hence any subsequent photonic crystals. Post centrifuge and replace, the morphology of the centrifuged particles is more uniform than that of the original particles, which has been evaluated using SEM micrographs. The face-centered cubic (FCC) structures observed on the surface of the photonic crystal films have also been imaged using a SEM. A linear equation for the prediction of the SNP diameters for a given initial amount of ethanol is proposed based on the centrifuged SNP diameters. The particle diameter measurements for the new equation were recorded using a DLS instrument. The dispersion of the SNPs was also recorded using DLS. The morphology of the surface of the particles has been confirmed using TEM micrographs.

Open image in new window

Graphical abstract ?

     

Heat transfer from a hot moving steel plate by using Cu-Al layered double hydroxide nanofluid based air atomized spray

The current research is focused on the cooling of a hot moving steel plate by using air atomized spray cooling technique. A new type of coolant, Cu-Al LDH nanofluid, has been prepared and used for heat flux removal. Preparation method of nanofluid and its characteristics has been reported. The cooling effectiveness is reported in terms of cooling rate by varying the concentration of nanofluid in five levels. The results indicate that the cooling rate increases at very low concentration of LDH with respect to base fluid. However, beyond a certain concentration a decreasing trend of cooling rate has been observed.

Abbreviations: CHF: Critical heat flux; HTC: Heat transfer coefficients; LDH:Layered double hydroxide; TEM: Transmission electron microscopy.     

Broadband near-infrared fibers dispersion measurement using white-light spectral interferometry

A new technique for an experimental determination of the effective refractive index, group refractive index and dispersion of fibers in a broad near-infrared spectral range is presented. The method is based on a white-light spectral interference which utilizes an unbalanced Michelson interferometer. The effective refractive index is obtained by a direct fitting the cosine function to the spectral interference pattern recorded by a low resolution spectrometer. The method has been tested in the spectral range of 1000-1700 nm both with standard telecommunication fibers and a sample of a photonic fiber. The accuracy of dispersion measurement () exceeds those from the previously reported near-infrared white-light spectral interference methods.     

The application of the linearized boltzmann transport equation to low energy atomic collisions in solids

A new method has been developed for the solution of the linearized Boltzmann equation far from thermal equilibrium. The solution has been applied to the interpretation of the slowing down of atoms with kinetic energies of a few hundreds of eV as obtained from measured -peak profiles, using the gamma ray induced doppler broadening (GRID) technique. From this solution, lifetimes of nuclear excited states can be extracted. These results are compared with the predictions of other models.     

Experimental study of a broad-band XeCl double-pass amplifier with SBS mirror

An oscillator-amplifier XeCl laser system has been used to experimentally investigate the effectiveness of a liquid stimulated Brillouin scattering (SBS) mirror for correcting the spatial aberrations of broad-band laser radiation in a double-pass amplifier. It has been found that the SBS amplifier performance is strongly dependent on the intensity I _p exciting the Brillouin medium. A good beam reconstruction has been attained at I _p1GW/cm², whereas highly aberrated output beams have been delivered by the SBS amplifier for I _p>1 GW/cm². By comparing the broad-band SBS amplifier performance to that of the same amplifier with a dielectrically coated flat mirror at one end, it has been found that the use of a SBS mirror is advantageous to obtain lower divergence output beams only for low energy pump beams (<1 mJ).     

Power transients in time-division multiplexed discrete Raman fibre amplifier

In this paper, the effect of addition and/or dropping of wavelength-division multiplexed channels in an all-optical gain-clamped time-division multiplexed (TDM) pumped discrete Raman fibre amplifier (DRFA) has been investigated experimentally and analyzed by numerical simulation. Nineteen signals were amplified in a counter-directionally pumped DRFA consisting of long dispersion compensating fibre. Light of eight DFB lasers was square-wave modulated at to imitate channel addition/dropping and combined with signals from five 10 Gigabit Ethernet transmitters and with six continuous wave signals. Power transients of the surviving channels caused by cross-gain modulation of the DRFA were monitored at the output of the amplifier. All-optical feedback loop was implemented in the form of a ring laser. Theoretical analysis of the TDM-pumped DRFA was based on numerical solution of coupled propagation equations for backward propagating pumps, signals, and both forward and backward propagating spectral components of amplified spontaneous emission powers.     

Analysis and simulation on the performance of a gyro-penitron amplifier

A cylindrical waveguide gyro-peniotron amplifier is analysed theoretically in this paper. By a ballistic method and directly using the energy conservation in the beam-wave system, a set of interaction equations for the cylindrical waveguide gyro-peniotron amplifier is derived. Computer simulation shows that under defined conditions an interaction efficiency of 47%, an output power of 240kW at a gain of 22. 5 dB and a 3 dB-instantaneous bandwidth of 1. 9% for a central frequency of 35GHz, TE₀₃ mode gyro-peniotron amplifier could be achieved. The influence of the operation parameters on the amplifier performance is also presented.     

Qualitative and quantitative analysis of lignite coal and its ash samples taken from Soma-Darkale region

Qualitative and quantitative analysis of lignite coal and its ash samples taken from Soma-Darkale region in city of Manisa (Turkey) have been performed by using the EDXRF technique. Samples are prepared from powder sifted by 200 mesh sieve.The elements Fe and Ba in the samples are analysed using the standard addition method. The samples were excited by gamma rays emitted from radioisotope source. The characteristic K X-rays emitted from samples were counted by means of Si(Li) detector which has a resolution at .     

Investigation and spectral analysis of the plasma-induced ablation mechanism of dental hydroxyapatite

Experiments on the ablation of dental substance performed with picosecond laser pulses are reported for the first time. A mode locked Nd:YLF oscillator laser was used to generate 25 ps pulses at a wavelength of 1.053µm. These were seeded and amplified to pulse energies up to 1 mJ in a regenerative amplifier laser at repetition rates up to 1 kHz. Very precise cavities were ablated in the enamel of extracted human teeth by mounting the probes onto a computer controlled 3D translation stage. Scanning electron microscopy and dye penetration tests were performed there-after. In contrast to longer pulse durations, picosecond pulses ablate with no signs of thermal damage, if the laser pulses are spatially distributed over the target. Definitions of the physical mechanisms plasma-induced ablation and photodisruption are given. Furthermore, the generated plasma spark has been spectroscopically analyzed. Excitations of calcium and sodium have been observed. From the spectra, the plasma temperature and free electron density could be estimated. By converting part of the laser energy into the second harmonic using a LiNbO₃ crystal, a reference amplitude was achieved for the spectra. With this reference signal, a clear distinction could be made between spectra obtained from healthy and caries infected teeth, thus enabling a better control of caries removal in the near future.